Method of bonding a chrome steel to a fibre composite

ABSTRACT

A method of bonding a high chrome steel to a fiber composite. An epoxy primer is applied to the high chrome steel, the primer is dried, and thereafter the high chrome steel and the fiber composite are adhered together by using an epoxy adhesive. The invention also includes bonded joints that are made by the method and bonded assemblies having the bonded joint. A joint made using this bonded method has a high torque-to-failure, even after exposure to extreme ambient conditions of high temperature and high relative humidity. The bonding method is particularly suitable for use in bonding high chrome steel and fiber composite components for use in the aerospace industry.

[0001] The present invention relates to a method of bonding, and hasparticular reference to a method of bonding high chrome steel to a fibrereinforced composite material. The invention also comprehends a bondedjoint formed using the method of the present invention, and a bondedassembly comprising a high chrome steel component and a fibre compositecomponent that are bonded together using the bonding method of thepresent invention.

[0002] In a particular aspect, the present invention provides a rotarydrive shaft comprising a carbon fibre composite tube and a high chromesteel flange that are bonded together using a bonding method of thepresent invention.

[0003] Composite rotary drive shafts are widely used throughout theaerospace industry, and are employed, for instance, in the wings ofaircraft. WO-A-98/20263, the contents of which are included herein byreference, discloses such a rotary drive shaft and a method formanufacturing it. The shaft of WO-A-98/20263 comprises a carbon fibrecomposite tube that is fitted at one end with a titanium flange. Saidtitanium flange is bonded to the composite tube, and the joint is alsoriveted for reinforcement. Said titanium flange constitutes an expensivecomponent, and at the time of writing there is a general desire withinthe aerospace industry to reduce the unit cost of components foraircraft.

[0004] Those skilled in the art will know that aerospace components aresubjected to extreme ambient conditions in use. In particular a specialproblem in the aircraft industry is the high temperature and highrelative humidity of the operating conditions. In order to be qualifiedfor use on aircraft, components are therefore subjected to rigoroustesting under extreme conditions of temperature and relative humidity.

[0005] There is a general requirement to bond components together,rather than to use mechanical fixings such as rivets or bolts, becausethe latter tend to work loose and/or fail through repeated stress-relaxcycles. However, it will be appreciated that adhesive bonding may becomevulnerable to attack by moisture. Furthermore, some adhesively bondedjoints may fail through differences in the coefficients of thermalexpansion of the adherends. Accordingly, there is an ongoing need tofind new methods and materials for bonding components for use in theaerospace industry which are subject to high temperature and highrelative humidity conditions.

[0006] The use of carbon fibre component materials is preferred withinthe aerospace industry because of the lightness of such components andtheir high strength and rigidity. However, the bonding of compositecomponents to other components represents an area of particulardifficulty, because moisture is able penetrate a joint not only alongthe bond line, but also through the composite itself.

[0007] According to one aspect of the present invention there isprovided a method of bonding a first high chrome steel adherend to asecond fibre composite adherend, which method comprises the steps ofapplying an epoxy primer to said first adherend, causing or allowingsaid primer to dry and thereafter adhering the first and secondadherends together using an epoxy adhesive.

[0008] In another aspect of the present invention there is provided abonded joint comprising a first high chrome steel adherend and a secondfibre composite adherend, which first and second adherends are bondedtogether using the method of bonding of the present invention.

[0009] In yet another aspect of the present invention there is provideda bonded assembly comprising a first high chrome steel component and asecond fibre composite component, which first and second components arebonded together using the method of bonding of the present invention.

[0010] In a preferred aspect of the present invention, said bondedassembly comprises a rotary drive shaft comprising a fibre compositetube and a high chrome steel flange, which tube and flange are bondedtogether using the bonding method of the present invention.

[0011] Preferably said tube is formed with at least one, and preferablytwo, flared ends, each of which is deformed to form an integrant flangeportion having an annular bonding face as described in WO-A-98/20263.Said bonding face is bonded to said steel flange using the bondingmethod of the present invention.

[0012] Said high chrome steel flange represents a significantly cheapercomponent than the titanium flange of the prior art. Optionally, thejoint between the tube and flange may be reinforced by rivets, bolts orany other suitable mechanical fixings.

[0013] Surprisingly, it has been found that the bonding method of thepresent invention allows a high chrome steel adherend to be bonded to afibre composite adherend to provide a joint which has a hightorque-to-failure, even after exposure to extreme ambient conditions ofhigh temperature and high relative humidity.

[0014] Said high chrome steel adherend may comprise a aerospace gradesteel. Specifically, the steel may comprise 12% to 20% by weightchromium, preferably 15% to 19% wt. Said steel may further comprise 0%to 8% wt nickel, preferably 2% to 6% wt. Typical additions of chromiumand nickel are 17% wt and 4% wt respectively. Thus, said first highchrome steel adherend may comprise a 17/4 grade steel.

[0015] Said second fibre composite adherend may comprise anaerospace-grade fibre-reinforced composite material comprising au epoxyresin. Preferably, said composite will comprise a carbon fibrereinforced composite material, although it is envisaged that thecomposite may be reinforced with any suitable fibres known in the art. Aspecific carbon fibre composite material suitable for use as the secondfibre composite adherend is the material 6376cHTA(12K-5-35) which iscommercially available from Hexcal Corporation.

[0016] Said first adherend is preferably pre-treated by abrading oretching before the primer is applied. Such pre-treatment may serve toremove contaminants or weak boundary layers and/or alter the surfacechemistry or surface morphology of the metal or any metal oxide surfacelayer(s).

[0017] Said first high chrome steel adherend may be decreased to removecontaminants before abrading or etching is carried out. A suitableabrasion agent is particulate aluminium oxide, for example 60 meshalumina in a water vehicle, which may be wet-blasted onto the firstadherend. Preferably, the water used for such wet-blasting is free ofanti-corrosion agents.

[0018] In some instances, etching ray be preferable to abrading, and asuitable etching solution comprises iron (III) chloride and hydrochloricacid. Such an etching solution will be familiar to those skilled in thearts Alternative etching methods comprise an oxalic acid/sulfuric acidetch, or etching using hydrochloric acid.

[0019] Said second adherend may be degreased before application of theadhesive. In some embodiments the second adherend will be protected by astrippatte film of the kind well known to those skilled in the art.

[0020] Said second adherend may also be pre-treated if and wherenecessary by abrading, particularly light abrading, to removecontaminants and/or loose or weakly bound material. Light abrasion maybe effected using, for example, a suitable grit paper, followed bydecreasing. A suitable grit paper is 120 mesh carborundum grit paper.

[0021] Said epoxy primer will advantageously have a rheology andchemical composition that is selected to match the morphology andsurface chemistry of the first adherend, to provide optimum protectionof the pre-treated surface, and to be compatible with the adhesive. Saidprimer may be solvent based, but is preferably water based, and may havea solids content of 5-15% wt. typically about 10% wt. Said primer maycomprise a simple surface protection solution. Advantageously however,the primer comprises a hardening agent. In some embodiments of theinvention, the primer comprises an anti-corrosion agent. Saidanti-corrosion agent may be any suitable such agent known to thoseskilled in the art such, for example, as a zinc salt anti-corrosionagent. Preferably however the anti-corrosion agent comprises a chromatesalt such, for example, as strontium chromate.

[0022] In one embodiment, the primer comprises a dispersion of an epoxyresin or mixture of epoxy resins in water. Said dispersion may furtherincorporate an epoxy curing agent. In particular, said curing agent maybe provided as a distinct phase. Said epoxy resin or mixture of epoxyresins may comprise one or more chain-extended, solid glycidyl ethers ofphenols, such as resorcinol and the bisphenols, such as bisphenol A,bisphenol F, and others familiar to the man skilled in the art.Alternatively, said epoxy resin or mixture of epoxy resins may compriseone or more of the solid glycidyl derivatives or aromatic amines andaminophenols, such as N,N,N′N′-solid DGEBA resins.

[0023] Preferably, said curing agent is substantially water insoluble,and is solid at room temperature. Said curing agent may comprise anaromatic amine curing agent such as 4,4′-diaminodiphenylmethane; 3,3′-or 4,4′-diaminodiphenyloxide; 3,3′- or 4,4′-diaminodiphenylsulfide; or3,3′- or 4,4′-diaminodiphenylketone. In particular, said curing agentmay comprise 4,4′-[1,4-phenylene(1-methylethylidene)]-bis(benzeneamine).However, various other solid diamine curing agents of the type wellknown to the man skilled in the art may also be used.

[0024] Said primer may further comprise a toughening agent, such as, forexample, an elastomer. Advantageously, said primer may further comprisean anti-corrosion additive or mixture of anti-corrosion additives. Saidanti-corrosion additive may comprise a chromate salt, for examplestrontium chromate, barium chromate, zinc chromate or lead chromate.Alternatively, said anti-corrosion additive may comprise a non-chromatecorrosion inhibitor, such as zinc phosphate, zinc molybdate or SICORINRZ available from BASF AG, Ludwigshafen, Germany. In some embodimentsthe primer may be free of any anti-corrosion additives.

[0025] The preparation of a suitable primer is described in U.S. Pat.No. 5,461,090, the contents of which are incorporated herein byreference.

[0026] Specifically, said primer may be BR®6747-1-A which iscommercially available from Cytec Industries, Inc. The CAS Registrynumber for this substance is 192390-61-7. BR®6747-1-A is a 100% waterbased corrosion inhibiting primer comprising a one part modified epoxychromate primer which contains substantially no volatile organiccompounds. A chromate-free equivalent of BR®6747-1-A may alternativelybe used.

[0027] Said primer may be air-dried or dried in an oven. In someembodiments the primer may be pre-cured, usually in an oven, beforeapplication of the adhesive.

[0028] Pre-curing of the primer may be advantageous because it may serveto localise functional ingredients of the primer (such as theanti-corrosion agent) juxtaposed the first adherend.

[0029] Said epoxy adhesive may be a film adhesive, a paste adhesive orany other suitable form of adhesive. Where a film adhesive is used, itmay comprise a suitable reticulated carrier (US: “support”) or it may beunsupported.

[0030] Said carrier is preferably formed from a polymeric plasticsmaterial such as nylon or polyester, with polyester being especiallypreferred. Said carrier may comprise a woven, knitted or needled fabric.Preferably, said fabric is formed from monofilamentis fibres, so as toavoid wicking of water into the bond.

[0031] Preferably the adhesive is a modified epoxy adhesive. A suitableadhesive is that which is available commercially from Cytec EngineeringMaterials Inc under the trade name FM300K, particularly FM300K.05 (250g/m²) . FM 300K is an epoxy resin adhesive film which may include 0-0.5%titanium dioxide. The CAS Registry number for FM 300K is 71210-48-5.

[0032] Said adhesive may be applied to the first adherend or the secondadherend, or both. The adhesive is then cured in an oven or autoclave.

[0033] The second composite adherend may be used in a pre-cured oruncured state. Advantageously, the second adherend is used uncured, andafter application of the primer to the first adherend (and optionallythe second adherend) and application of the adhesive, the compositematerial is co-cured with the primer and adhesive.

[0034] The present invention thus provides a method of bonding a highchrome steel component to a carbon fibre composite component which hassufficient torque-to-failure under conditions of high moisture and hightemperature to make it suitable for use in the aerospace industry.

[0035] Following is a description by way of example only with referenceto the accompanying drawings of embodiments of the present invention.

[0036] In the drawings:

[0037]FIG. 1 is a side view, partly in cross-section, of one end of arotary power transmission shaft in accordance with the presentinvention.

[0038]FIG. 2 is an end view of the shaft of FIG. 1.

EXAMPLES Adherends

[0039] Stainless steel (AMS 5643 (H1025), 17/4, 54 mm dia) and fibrereinforced epoxy composite (Fibredux 6376, 75×753 mm) components wereused. The epoxy composite was supplied with a protective strip ply thatwas removed before use.

Adhesive

[0040] Cytec FM300K—this is a modified epoxy film adhesive with apolyester carrier and supplied by Cytec Engineering Materials Inc. CASRegistry no. 71210-48-5. The grade used was FM300K.05 (250 g/m²)

Primer

[0041] BR®6747-1-A, 100% water based corrosion inhibiting primer. Thisis a 100% water based corrosion inhibiting primer comprising a one partmodified epoxy chromate primer which contains substantially no volatileorganic compounds. CAS Registry no. 192390-61-7. This primer was sprayedon stainless steel adherends after pre-treatment.

Surface Pre-Treatment (After Removing the Strip Ply)

[0042] The epoxy composite components were first degreased usingLotoxane® wipes then abraded with 120 alumina grit and degreased. Anumber of different pre-treatments (See Table 1 below) were used toprepare the surfaces of the stainless steel adherend before bonding.Details of each pre-treatment are as follows:

Ferric Chloride Hydrochloric Acid Etch

[0043] The following etch solution was used to treat stainless steelcomponents.

[0044] a) 50/50 (w/w) of Iron (III) chloride and 35-37% HCl

[0045] b) Temperature of solution was maintained at 20-23° C.

[0046] c) Etched for 10 minutes

[0047] d) washed with deionised water

[0048] e) Dried at room temperature,

Wet Blasting

[0049] Wet blasting was carried out using 60 mesh aluminium oxide at anair pressure of 401 bf/in² for a duration of approximately 2-3 seconds.

Primer

[0050] The primer was used as follows:

[0051] BR®6747-1-A thin coating (3-40 m) of this primer was appliedusing a spray gun. The coated specimen was first allowed to dry at roomtemperature and then the primer was cured at 120° C. for 60 minutes. Agravity feed air pressure spray gun (M21G) supplied by Kremlin) was usedto spray the Cytec primer. This gun works on the principle ofconventional spraying which mixes the primer with air as it leaves thegun. The primer comes out of the nozzle under low pressure and issprayed by a fan or compressed air coming from the aircap at around 3-6bar. On pulling the trigger, firstly the air is liberated by the airvalve, and then the product is emitted by the needle. The whole assemblyof the aircap, the needle and the nozzle is called the projector. Theprimer was sprayed at a pressure of 4 bar. The specimens were coated bya single pass or spraying to give the required thickness as describedearlier.

Joint Preparation

[0052] The joints were prepared using the stainless steel and fibrereinforced epoxy composite components after various pre-treatments orprimer/adhesive combinations. The preparation conditions and number ofspecimens required to assess the joint strength before and after ageingare shown in Tables 1 and 2 below. A jig was specifically designed andconstructed for assembly and bonding of the components. The base of thejig consisted of a large mild steel washer (65 mm diameter) with an M6clearance hole. A length of 6 mm studding was pet through the hole and aM6 nut as well as a locking nut were placed on the underside of thewasher. The composite part of the bonding component was positioned inplace and this was followed by placement of the film adhesive andstainless steel component. A calibrated music wire spring with a ratingof 20 N/mm was placed over the studding and this was held compressed bya washer and a nut. This type of spring allowed pressure of 2 bar to beapplied to the joint during cure. The whole assembly of each joint wasthen placed in an oven to cure.

Joint Assessment

[0053] The strength of each set of joints was determined by measuringthe torque-to-failure using a torque wrench as supplied by MHHEngineering. The torque wrench was equipped with 200-2000 Nm range and 1inch drive. A one to three quarter inch adapter was used to attach thetorque wrench to the specimen which also had a ¾ inch shaft. In order tohold the bonded components during testing a jig was constructed. The jigmainly consisted of a top and a bottom plate, with a steel jointlocation plate in between.

Durability Testing

[0054] The durability of the bonded joint in a hot and humid environmentwas studied, at a temperature of 70° C. and a relative humidity of 95%.Eighty-four epoxy composite/stainless steel bonded joints, as well asthirty epoxy composite components (75×75×3 mm plates), were placed inthe chamber. The epoxy composite test pieces were removed from thechamber and weighed three comes during the first week, and then once aweek for a period of 8 weeks. The percentage moisture uptake for eachindividual composite specimen was measured and recorded. Differentspecimens were exposed to four and eight weeks ageing at 70° C. and 85%RH as shown in Table 2 and then removed for testing. The specimens (seeTable 3 below) were also exposed at 70° C. and 100%RH after reaching1.2% moisture (10 days exposure) uptake. The torque-to-failure of thesejoints was also determined as shown in Table 3.

Assessment of Joints after Ageing

[0055] The results of four and eight weeks ageing of the joints at 70°C. and 85% RH are shown in Table 2.

[0056] The samples prepared after grit blasting and aged for eight weeksresulted in mean failure torque similar to those aged for four weeks.However, the result of four weeks ageing is only based on two joints andtherefore can not be statistically compared. Nevertheless, it should bementioned that these results shown there is little or no loss in failuretorque on additional four weeks ageing

[0057] A similar trend in failure torque was observed with specimenstreated with FeCl₃+HCl etch during four and eight weeks ageing. The meanfailure torque was 1025 Nm after eight weeks compared with 1150 Nm afterfour weeks ageing.

[0058] Compared with control values (Table 1) both pre-treatments showreductions in failure torque; 20% for grit blast, 26% for etchpre-treatment.

[0059] The grit blast pre-treatment shows little change in failure mode,with mixed cohesive failure in the adhesive and composite ply failure,indicating exposure effects may be limited to the adhesive and notinterfaces.

[0060] The etch pre-treated specimens show a reduction in compositefailure after exposure, with an increase in cohesive failure within theadhesive layer. This may be the result of water ingress into theadhesives either along the carrier/adhesive interface, or through thebulk of the adhesive or both. It is noteworthy that one specimen shows ahigh level of adhesion failure to the stainless steel interface.

[0061] The higher coefficient of variation value for surface exposedetch specimens (15%) compared with exposed grit blast specimens (12%)follows the trend in control specimens results (grit blast 10%, etch19%).

[0062] The differences in mean strength after exposure are probably notsignificant (grit blast 1085 Nm, etch 1025 Nm).

Ageing After 1.2% Moisture Uptake

[0063] The results of ageing of joints at 70° C. and 100% RH to reach1.2% moisture uptake after 10 days is shown in Table 3.

[0064] With only one sample for the two treatments (grit blast and etch)used with Cytec EM300k/6747-1 definitive conclusions cannot be drawn.However, from inspection of the results both failure torque and failuremode are similar to the results of both 4 and 8 weeks high humidityexposure. It should be noted however, that for the 1.2% moisture uptakeequilibrium test, exposure conditions (70° C./100%RH) differed slightlyfrom the 4 and 8 week exposure conditions (70° C./85%RH). TABLE 1 SteelPre- Failure Specimen No treatment Adhesive Primer Torque (Nm) FailureMode Comments 1 a Grit Blast Cytec Cytec 1500  80% Co.Adh 20% 1/2 plyfail  b FM300K BR ®6747-1 1250  50% Co.Adh to Steel/Comp 50% 1/2/3 plyfail  c  700 Visible crack in glue-line edge - discounted  d 1400  50%Co.Adh to Steel/Comp 50% 1/2/3 ply fail  e 1250  75% Co.Adh toSteel/Comp 25% 1/2 ply fail 2 a FeCl₃ + HCl Etch Cytec Cytec 1750  90%Co.Adh to Steel/Comp 10% 1/2 ply fail  b FM300K BR ®6747-1 1550 100%co.Adh to Steel/Comp <2.5% 1/2 ply fail  c 1250  75% co.Adh toSteel/Comp 25% 1/2/3 ply fail  d 1100  20% Co.Adh to Steel/Comp 80%1/2/3 ply fail  e 1250  10% Co.Adh  0% 1/2/3 ply fail

[0065] TABLE 2 Ageing Specimen Time Steel Pre- Failure No (weeks)treatment Adhesive Primer Torque (Nm) Failure Mode Comments 1 g 4 GritBlast Cytec Cytec  700  70% Co.Adh to S/C 30% Comp. Fail  o FM300KBR ®6747-1 1150  90% Co.Adh to S/C 10% Comp. Fail 2 g 4 FeCl₃ + HClCytec Cytec 1150  40% Co.Adh in S/C 60% Comp. Fail  o Etch FM300KBR ®6747-1 1150  50% Co.Adh to S/C 50% Comp. Fail 1 f 8 Grit blast CytecCytec  800 100% Co.Adh to S/C  h FM300K BR ®6747-1 1150  80% Co.Adh toS/C 20% Comp. fail  i 1200  30% Co.Adh to S/C 70% Comp. fail  j 1150 70% Co.Adh to S/C 30% Comp. fail  k 1100  75% Co.Adh to S/C 25% Comp.fail  l 1100  75% Co.Adh to S/C 25% Comp. fail  m  950  40% Co.Adh toS/C 60% Comp. fail  n 1200  90% Co.Adh to S/C 10% Comp. fail  p 1050 80% Co.Adh to S/C 20% Comp. fail  q 1150  80% Co.Adh to S/C 20% Comp.fail 2 f 8 FeCl₃ + HCl Cytec Cytec 1200  80% Co.Adh. to S/C 20% Comp.fail  h Etch FM300K BR ®6747-1 1150  80% Co.Adh to S/C 20% Comp. fail  i1050 100% Adh to S/C Slight “snicking” noise daring test  j 1150  60%Co.Adh to S/C 40% Comp. fail  k 1000 100% Co.Adh to S/C Slight“snicking” noise during test  l 1200  90% Co.Adh to S/C 10% Comp. fail m  850  60% Co.Adh to S/C 40% Comp. fail  n 1000 100% Co.Adh to S/CSample slipped in text fixture  p  800 100% Co.Adh to S/C Sample slippedin text fixture  q  850 100% Co.Adh to S/C

[0066] Speci- Steel Failure men Pre- Torque Failure No treatmentAdhesive Primer (Nm) Mode Comments 1r Grit Cytec Cytec 1000  97% 3%Comp. Blast FM300K BR ® Co.Adh fail 6747-1 to S/C 2r FeCl₃ + Cytec Cytec1000 100% HCl Etch FM300K BR ® Co.Adh 6747-1 to S/C

[0067]FIG. 1 shows one end of a rotary power transmission shaft that issuitable for use in driving the flaps in aircraft wings. The other endof the shaft may, in some embodiments, be substantially the same as theend shown, or it may be different. Said one end of the shaft comprisesan end plate (10) that is made as one piece from a high chrome steelsuch, for example, as AMS 5643 (H1025)17/4. Said end plate (10)comprises a generally annular portion (12) having a central aperture(14), and a tubular spigot portion (16) that is positionedconcentrically with the aperture (14) and meets the annular portion (12)around the edge of the said aperture (14). As can be seen from FIG. 1,the end plate (10) is chamfered on its inner surface at (18) where theannular portion meets the spigot portion around the aperture (14). Theopposing outer surface of the end plate is curved where the annularportion (12) and spigot portion (16) meet to provide a smooth transitiontherebetween. The outer surface of the spigot portion (16) is rebated at(20).

[0068] Said spigot portion (16) of the end plate (10) is spigotted inone end of a tubular mandrel (30) that is made of carbon fibrereinforced plastics material. The spigot portion (16) forms a snug fitwithin the end of the mandrel (30), and the two are mutually located bythe rebate (20).

[0069] Said tubular mandrel (30) lines the inner surface of a tube (32)that has an integrant annular flange (34) at the said one end. Saidflange (34) protrudes beyond the end of the mandrel (30) and liescontiguous the outer surface of the end plate (10) as shown in thefigure. The flange (34) of the tube (32) is radially coterminous withthe annular portion (12) of the end plate (10) and is fixedly securedthereto using an adhesive bond in accordance with the invention.

[0070] The rotary transmission shaft shown in FIGS. 1 and 2 can beassembled substantially in the same way as described in WO-A-98/20263 bywinding carbon fibre tape impregnated with thermosetting resin onto themandrel (30) using a conventional tape winding machine fitted withsuitable shaped centres for holding the mandrel. The outer surface ofthe end plate (10) that is to mate with the flange (34) of the tube (32)is pre-treated by abrasion in the manner described above, and a layer ofan epoxy primer such as BR® 6747-1-A available from Cytec EngineeringMaterials Inc is applied. An epoxy film adhesive such as Cytec FM300K®is then applied to the outer surface of the end plate (10), and saidsurface is then offered up to the annular flange (34) of the tube (32)such that the end plate (10) is spigotted within the mandrel (30). Theassembly is then fitted with a suitably shaped mould and cured within anautoclave as described in WO-A-98/20263 to cure the primer, adhesive andtube (32). The bonded assembly is then removed from the autoclave andthe edges finished.

[0071] Optionally, the joint between the end plate (10) and the tube(32) may he reinforced by rivets or other suitable fixings. Forinstance, in the embodiment shown in FIGS. 1 and 2 the bonded flangeportion (12) of the end plate and flange (34) of the tube (32) aredrilled at a plurality of circumferentially spaced locations, in thiscase four, to form holes (22). In the embodiment shown, each of theholes (22) is formed in a respective mesa (24) which protrudes from theface of the end plate (10) that is opposite to the face bonded to thetube (32). Each of the holes (22) accommodates a tubular bush (notshown) which constitutes a rivet for holding the composite and steelcomponents together should the adhesive bond fail.

1. A method of bonding a first high chrome steel adherend to a secondfibre composite adherend, which method comprises the steps of applyingan epoxy primer to said first adherend, causing or allowing said primerto dry and thereafter adhering the first and second two adherendstogether using an epoxy adhesive.
 2. A method as claimed in claim 1 ,wherein said high chrome steel comprises an aerospace grade steel.
 3. Amethod as claimed in claim 1 , wherein said high chrome steel comprises12 to 20% by weight of chromium.
 4. A method as claimed in claim 1 ,wherein said steel comprises 0 to 8% by weight nickel.
 5. A method asclaimed in claim 1 , wherein said first high chrome steel adherendcomprises a 17/4 grade steel.
 6. A method as claimed in claim 1 ,wherein second fibre composite adherend comprises an aerospace-gradefibre-reinforced composite material comprising an epoxy resin.
 7. Amethod as claimed in claim 1 , wherein said composite comprises a carbonfibre-reinforced composite material.
 8. A method as claimed in claim 1 ,wherein said second fibre composite adherend comprises the material6378cHTA(12K-5-35) which is commercially available from HexcelCorporation.
 9. A method as claimed in claim 1 , wherein said firstadherend is pre-treated by abrading or etching before the primer isapplied.
 10. A method as claimed in claim 9 , wherein said first highchrome steel adherend is degreased to remove contaminants beforeabrading or etching.
 11. A method as claimed in claim 1 , wherein saidsecond adherend is degreased before application of said adhesive.
 12. Amethod as claimed in claim 1 , wherein said second adherend ispre-treated by abrading to remove contaminants and/or loose or weaklybound material.
 13. A method as claimed in any claim 1 , wherein saidepoxy primer has a rheology and chemical composition that is selected tomatch the morphology and surface chemistry of the first adherend.
 14. Amethod as claimed in claim 1 , wherein said primer is water based,having a solids content of 5-15% weight.
 15. A method as claimed inclaim 1 , wherein said primer comprises a hardening agent.
 16. A methodas claimed in claim 1 , wherein said primer comprises an anti-corrosionagent.
 17. A method as claimed in claim 16 wherein said anti-corrosionagent comprises a zinc salt and/or a chromate salt.
 18. A method asclaimed in claim 1 , wherein said primer comprises an aqueous epoxyresin dispersion which contains as a distinct phase a solid epoxy curingagent.
 19. A method as claimed in claim 18 , wherein said aqueous epoxyresin dispersion comprises one or more solid epoxy resins.
 20. A methodas claimed in claim 19 , wherein one or more of said solid epoxy resinscomprises a glycidyl ether of a phenol or a glycidyl derivative of anaromatic amine or aminophenol.
 21. A method as claimed in claim 18 ,wherein said curing agent is substantially water insoluble and solid atroom temperature.
 22. A method as claimed in claim 18 , wherein saidcuring agent comprises an aromatic amine or diamine curing agent.
 23. Amethod as claimed in claim 18 , wherein said primer further comprises achromate or non-chromate corrosion inhibitor.
 24. A method as claimed inclaim 1 , wherein said primer is BR®6747-1-A which is commerciallyavailable from Cytec Industries Inc.
 25. A method as claimed in claim 1, wherein said primer consists of a chromate-free equivalent toBR®6747-1-A.
 26. A method as claimed in claim 12 wherein said primer ispre-cured.
 27. A method as claimed in claim 1 , wherein said epoxyadhesive comprises a film adhesive or paste adhesive.
 28. A method asclaimed in claim 1 , wherein said adhesive comprises a carrier that isformed from a polymeric plastics material.
 29. A method as claimed inclaim 1 , wherein said adhesive is a modified epoxy adhesive.
 30. Amethod as claimed in claim 1 , wherein said adhesive is FM300K.05 whichis available from Cytec Engineering Materials Inc.
 31. A method asclaimed in claim 1 , wherein the second adherend is used uncured and,after application of said primer to the first adherend and applicationof the adhesive, the composite material is co-cured with the primer andadhesive.
 32. A bonded joint comprising a first-chrome steel adherendand a second fibre composite adherend, which first and second adherendbonded together using a method of bonding as claimed in claim 1 .
 33. Abonded assembly comprising a first high chrome component and a secondfibre composite component, which first and second components are bondedtogether using a method of bonding as claimed in claim 1 .
 34. A bondedassembly as claimed in claim 33 , which assembly comprises a rotarydrive shaft comprising a fibre composite tube and high chrome steelflange, which tube and flange are bonded together using said bondingmethod.
 35. An assembly as claimed in claim 34 , wherein said tube isformed with at least one flared end, and the or each tube is deformed toform an integrant flange portion having an annular bonding face.
 36. Anassembly as claimed in claim 35 , wherein the joint between the tube andthe flange is reinforced by rivets, bolts or any other suitablemechanical fixings.